U.S. patent number 5,740,880 [Application Number 08/568,729] was granted by the patent office on 1998-04-21 for speed tracking of induced armature field in electric power assisted steering.
This patent grant is currently assigned to Ford Global Technologies, Inc.. Invention is credited to John Michael Miller.
United States Patent |
5,740,880 |
Miller |
April 21, 1998 |
Speed tracking of induced armature field in electric power assisted
steering
Abstract
Electric motors having controllable induced armature fields,
such as induction motors and synchronous reluctance motors, are
used in power assisted steering systems for motor vehicles. Power
is conserved by tailoring induced armature fields or rotor flux in
accordance with the speeds of motor vehicles including the power
assisted steering system. In particular, one or more flux programs
or maps are provided for the power assisted steering system with
the flux map or program being accessed or addressed by means of the
vehicle speed. During low speed operation of the motor vehicle, for
example to perform parking maneuvers where speeds are near zero and
steering forces are near or at maximum, the rotor flux is
programmed to maximum. For high speed operation, such as highway
and rural motor vehicle operation, the rotor flux is programmed to
a low value so that internal loss mechanisms in the power assist
motor and motor controller are minimized yet provide sufficient
rotor flux to meet steering needs such as lane changes, obstacle
avoidance and the like. Various transition speeds and flux
transition curves provide smooth transitions between high flux
levels and low flux levels.
Inventors: |
Miller; John Michael (Saline,
MI) |
Assignee: |
Ford Global Technologies, Inc.
(Dearborn, MI)
|
Family
ID: |
24272482 |
Appl.
No.: |
08/568,729 |
Filed: |
December 7, 1995 |
Current U.S.
Class: |
180/446; 318/718;
318/805; 318/806; 318/825 |
Current CPC
Class: |
B62D
5/046 (20130101); B62D 5/0463 (20130101) |
Current International
Class: |
B62D
5/04 (20060101); H02P 007/00 (); H02P 005/40 ();
B62D 005/04 () |
Field of
Search: |
;364/424.051,424.052,424.053 ;180/400,443,444,445,446
;318/718,721,825,823,806,805,802,809,432 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
I Takahashi & T. Noguchi, A New Quick-Response and
High-Efficiency Control Strategy of an Induction Motor, IEEE
Transactions on Industry Applications, vol. IA-22, No. 5, Sep./Oct.
1986. .
P. Krein, F. Disilvestro, I. Kanellakopoulos, J. Locker,
"Comparative Analysis of Scalar and Vector Control Methods for
Induction Motors," IEEE Transactions on Industry Applications,
1993..
|
Primary Examiner: Johnson; Brian L.
Assistant Examiner: Savitt; Gary
Attorney, Agent or Firm: May; Roger L. Dixon; Richard D.
Claims
What is claimed is:
1. A motor driven power assisted steering system for a motor
vehicle comprising:
an electric motor with a controllable induced armature field;
a coupler mechanism for coupling an output shaft of said electric
motor to steering gear of a motor vehicle;
a vehicle speed sensor coupled to said motor vehicle for detecting
the operating speed of said motor vehicle and for generating
representative speed signals; and
a motor controller responsive to said speed signals for controlling
the induced armature field of said electric motor as a function of
the operating speed of said motor vehicle, said motor controller
operating along a flux versus vehicle velocity curve to reduce
power consumed by said electric motor, said flux versus vehicle
velocity curve having a maximum value for a range of low vehicle
speeds, a minimum value for a range of high vehicle speeds and a
smooth transition between said range of low vehicle speeds and said
range of high vehicle speeds.
2. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 1 wherein said electric motor comprises
an induction motor.
3. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 1 wherein said electric motor comprises
a synchronous reluctance motor.
4. A motor driven power assisted steering system for a motor
vehicle comprising:
an electric motor with a controllable induced armature field;
a coupler mechanism for coupling an output shaft of said electric
motor to steering gear of a motor vehicle;
a vehicle speed sensor coupled to said motor vehicle for detecting
the operating speed of said motor vehicle and for generating
representative speed signals; and
a motor controller responsive to said speed signals for controlling
the induced armature field of said electric motor as a function of
the operating speed of said motor vehicle, said motor controller
including a flux versus velocity curve comprising at least two
piecewise continuous portions wherein the velocity of said flux
versus velocity curve is the velocity of said motor vehicle.
5. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 4 wherein said flux versus velocity
curve comprises an initial portion extending between a zero
velocity and a first velocity and passing through a fixed maximum
point of flux.
6. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 5 wherein said flux versus velocity
curve comprises a second portion wherein said flux decreases at a
first rate from said fixed maximum flux to a reduced intermediate
flux and extending between said first velocity and a second
velocity greater than said first velocity.
7. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 6 wherein said flux versus velocity
curve comprises a third portion wherein said flux is maintained at
substantially said reduced intermediate flux and extending between
said second velocity and a third velocity.
8. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 7 wherein said flux versus velocity
curve comprises a fourth portion wherein said flux decreases at a
second rate less than said first rate from said reduced
intermediate flux to a minimum flux and extending from said third
velocity to a fourth velocity.
9. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 8 wherein said flux versus velocity
curve comprises a fifth portion wherein said flux is maintained at
substantially said minimum flux and extending from said fourth
velocity for velocities greater than said fourth velocity.
10. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 9 wherein said first and second rates
are substantially linear.
11. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 10 wherein said first and third
velocities are approximately 5 mph and 15 mph, respectively.
12. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 10 wherein said first and third
velocities are approximately 15 mph and 45 mph, respectively.
13. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 10 wherein said first velocity is
within a range of approximately from 5 to 15 mph and said third
velocity is within a range of approximately from 15 to 45 mph,
respectively.
14. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 5 wherein said flux versus velocity
curve comprises a second portion wherein said flux decreases at a
defined rate from said fixed maximum flux to a minimum flux and
extending between said first velocity and a second velocity greater
than said first velocity.
15. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 14 wherein said flux versus velocity
curve comprises a third portion wherein said flux is maintained at
substantially said minimum flux and extending from said second
velocity for velocities greater than said second velocity.
16. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 15 wherein said first and second
velocities are approximately 5 mph and 15 mph, respectively.
17. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 15 wherein said first and second
velocities are approximately 15 mph and 45 mph, respectively.
18. A motor driven power assisted steering system for a motor
vehicle as claimed in claim 15 wherein said first velocity is
within a range of approximately from 5 to 15 mph and said second
velocity is within a range of approximately from 15 to 45 mph,
respectively.
19. A method for operating a power assisted steering system for a
motor vehicle, said power assisted steering system including an
electric motor having an output shaft coupled to steering gear of
said motor vehicle and a controllable induced armature field, said
method comprising the steps of:
detecting the operating speed of said motor vehicle;
generating speed signals representative of the operating speed of
said motor vehicle; and
controlling the induced armature field of said electric motor in
response to said speed signals as a function of the operating speed
of said motor vehicle by operating along a flux versus vehicle
velocity curve to reduce power consumed by said electric motor,
said step of controlling the induced armature field comprising the
steps of:
moving along a low speed portion of said flux versus vehicle
velocity curve extending from zero velocity to a first velocity and
passing through a maximum point of flux;
moving along an intermediate portion of said flux versus vehicle
velocity curve extending from said first velocity to a third
velocity which intermediate portion of said curve provides a smooth
transition between said low speed portion of said curve and a high
speed portion of said curve; and
moving along said high speed portion of said flux versus vehicle
velocity curve extending upward from said third velocity and
passing through a minimum point of flux.
20. A method for operating a power assisted steering system for a
motor vehicle, said power assisted steering system including an
electric motor having an output shaft coupled to steering gear of
said motor vehicle and a controllable induced armature field, said
method comprising the steps of:
detecting the operating speed of said motor vehicle;
generating speed signals representative of the speed of said motor
vehicle; and
controlling the induced armature field of said electric motor in
response to said speed signals as a function of a flux versus
velocity curve comprising at least two piecewise continuous
portions wherein said velocity of said flux versus velocity curve
is the velocity of said motor vehicle.
21. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 20 further comprising the step of
forming said flux versus velocity curve to extend between a zero
velocity and a first velocity and have an initial portion passing
through a fixed maximum flux.
22. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 21 further comprising the step of
forming said flux versus velocity curve to have a second portion
wherein said flux decreases at a first rate from said fixed maximum
flux to a reduced intermediate flux and extending between said
first velocity and a second velocity greater than said first
velocity.
23. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 22 further comprising the step of
forming said flux versus velocity curve to have a third portion
wherein said flux is maintained at substantially said reduced
intermediate flux and extending between said second velocity and a
third velocity.
24. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 23 further comprising the step of
forming said flux versus velocity curve to have a fourth portion
wherein said flux decreases at a second rate less than said first
rate from said reduced intermediate flux to a minimum flux and
extending from said third velocity to a fourth velocity.
25. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 24 further comprising the step of
forming said flux versus velocity curve to have a fifth portion
wherein said flux is maintained at substantially said minimum flux
and extending from said fourth velocity for velocities greater than
said fourth velocity.
26. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 25 further comprising the step of
setting said first and second rates to be substantially linear.
27. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 26 further comprising the step of
setting said first and third velocities to approximately 5 mph and
15 mph, respectively.
28. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 26 further comprising the step of
setting said first and third velocities to approximately 15 mph and
45 mph, respectively.
29. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 26 further comprising the step of
setting said first velocity within a range of approximately from 5
to 15 mph and said third velocity within a range of approximately
from 15 to 45 mph, respectively.
30. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 21 further comprising the step of
forming said flux versus velocity curve to have a second portion
wherein said flux decreases at a defined rate from said fixed
maximum flux to a minimum flux and extending between said first
velocity and a second velocity greater than said first
velocity.
31. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 30 further comprising the step of
forming said flux versus velocity curve to have a third portion
wherein said flux is maintained at substantially said minimum flux
and extending from said second velocity for velocities greater than
said second velocity.
32. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 31 further comprising the step of
setting said first and second velocities to approximately 5 mph and
15 mph, respectively.
33. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 31 further comprising the step of
setting said first and second velocities to approximately 15 mph
and 45 mph, respectively.
34. A method for operating a power assisted steering system for a
motor vehicle as claimed in claim 31 further comprising the step of
setting said first velocity within a range of approximately from 5
to 15 mph and said second velocity within a range of approximately
from 15 to 45 mph, respectively.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to power steering systems
using an electric motor to produce auxiliary steering force for
augmenting the torque applied to a steering wheel by an operator of
a motor vehicle and, more particularly, to the use of electric
motors having controllable induced armature fields, such as
induction motors and synchronous reluctance motors, and the control
of the induced armature fields in such motors to reduce the power
consumed by the power steering systems.
Electric power assisted steering (EPAS) is being developed to
improve steering control capabilities, reduce system costs and, at
least in part, to improve fuel economy over power assisted
hydraulic systems. A wide variety of electric motors are available
for use in EPAS ranging from permanent magnet brushed and
brushless, to switched and synchronous reluctance, to induction
motors. Physical size favors the permanent magnet motor while cost
favors the reluctance and induction motors. Smoothness of operation
also favors synchronous reluctance and induction motors since EPAS
should not introduce extraneous "noise" and vibration into the
steering wheel and switched reluctance motors tend to have more
torque ripple than desired for use in EPAS.
While cost and smoothness of operation favor synchronous reluctance
and induction motors over other motors available for EPAS,
synchronous reluctance motors and induction motors require the
provision of external power to energize or maintain the flux in the
armature or rotor of the motor. Accordingly, if synchronous
reluctance motors and/or induction motors are to be used for EPAS,
there is a need to reduce the energy consumed by these motors for
armature or rotor excitation.
SUMMARY OF THE INVENTION
This need is met by the invention of the present application
wherein electric motors having controllable induced armature
fields, such as induction motors and synchronous reluctance motors,
are used in power assisted steering systems for motor vehicles.
Power is conserved by tailoring induced armature fields or rotor
flux in accordance with the speed of a motor vehicle including the
power assisted steering system. In particular, one or more flux
programs or maps are provided for the power assisted steering
system with the flux map or program being accessed or addressed by
means of the vehicle speed. During low speed operation of the motor
vehicle, for example to perform parking maneuvers where speeds are
near zero and steering forces are near or at maximum, the rotor
flux is programmed to maximum. For high speed operation, such as
highway and rural motor vehicle operation, the rotor flux is
programmed to a low value so that internal loss mechanisms in the
power assist motor and motor controller are minimized yet provide
sufficient rotor flux to meet steering needs such as lane changes,
obstacle avoidance and the like. Various transition speeds and flux
transition curves provide smooth transitions between high flux
levels and low flux levels.
In accordance with one aspect of the present invention, a motor
driven power assisted steering system for a motor vehicle comprises
an electric motor with a controllable induced armature field. A
coupler mechanism couples an output shaft of the electric motor to
steering gear of a motor vehicle which includes a vehicle speed
sensor for detecting the operating speed of the motor vehicle and
for generating representative speed signals. A motor controller
responsive to the speed signals controls the induced armature field
of the electric motor as a function of the operating speed of the
motor vehicle. The electric motor may be an induction motor or a
synchronous reluctance motor.
It is, thus, a feature of the present invention to provide an
improved power assisted steering for motor vehicles wherein power
assistance is provided by electric motors having controllable
induced armature fields and to reduce power consumption in those
motors by programming induced armature fields or rotor flux in
accordance with the speeds of motor vehicles including the power
assisted steering systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an electric power assisted
steering (EPAS) system for a motor vehicle in accordance with the
present invention;
FIG. 2 is a graph of flux programs or maps and a steering torque
curve for the EPAS system of FIG. 1;
FIG. 3 is a schematic block diagram of a portion of the power
inverter and induction motor controller of FIG. 1;
FIG. 4 is a vector diagram illustrating the three phase stator
currents of FIG. 5;
FIG. 5 is a graph of three phase stator currents in an induction
motor; and
FIG. 6 is a graph illustrating two slip curves for the induction
motor of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Reference is now made to FIG. 1 which schematically illustrates an
electric power assisted steering (EPAS) system 100 including speed
tracking of an induced armature field in a motor 102 which performs
steering assistance. The motor 102 has a controllable induced
armature field, currently an induction motor is preferred and the
invention will be described with reference to an induction motor;
however, a synchronous reluctance motor can also be used in the
present invention. The system 100 includes a steering wheel 104
which is operatively connected to a pinion gear 106 via a steering
shaft 108. A torque sensor 110 is coupled to the steering shaft 108
to measure the torque Tstr applied to the steering wheel 104 by an
operator of the motor vehicle including the system 100.
The motor 102 includes an output shaft 112 which is coupled to the
steering shaft 108 via a gear set 114. The pinion gear 106, which
is driven by the steering shaft 108, engages and drives a linear
steering member or rack 116 which is connected to steerable wheels
(not shown) in a conventional manner.
A conventional vehicle speed sensor 118, coupled to a transmission
or one or more of the wheels of the vehicle including the system
100, generates vehicle speed information signals which are
connected to a vehicle power train controller 120. The power train
controller 120 processes the vehicle speed information signals to
generate speed dependent address signals for a flux map table 122
which includes at least one speed dependent flux program or map for
operation of the motor 102 as will be described.
The controller 120 also generates a power steering command signal
which operates a relay 124 to activate or deactivate the EPAS
system 100 by connecting or disconnecting power from a power
inverter and induction motor controller 126. This allows the EPAS
system 100 to operate with the ignition off or the engine stalled
unlike hydraulic systems which it replaces. In addition, it permits
the vehicle power train controller 120 to disconnect the EPAS
system 100 in the event of failure within the system 100.
The power inverter and induction motor controller 126 includes
sensors for determining the flux waveform .lambda.dr in the rotor
of the motor 102, the quadrature current Iq and/or for monitoring
the speed .omega.r and the torque Tr of the output shaft 112 of the
motor 102 via sensors associated with the gear set 114 or otherwise
associated with the motor 102 or the output shaft 112. The flux
waveform .lambda.dr is passed to an electromagnetic (EM) torque
calculator 128 and a proportional-integral-derivative (PID) flux
regulator 130 via a conductor 132. The PID flux regulator generates
a flux command signal I.sub.d.sup.e*.
The EM torque calculator 128 also receives the quadrature current
Iq via a conductor 134. To determine the EM torque, the torque
calculator 128 executes the function:
where P is the number of poles of the motor 102, Lm is the
magnetizing inductance of the motor 102 and Lr is the rotor
inductance of the motor 102. No friction effects are included in
the Em torque calculator.
The torque Tstr applied to the steering wheel 104 by an operator of
the motor vehicle including the system 100 as sensed by the torque
sensor 110 is passed to a summer 136. The summer 136 also receives
the EM torque Tem calculated by the EM torque calculator 128 and
subtracts the EM torque Tem from the sensed torque Tstr. The
resulting error signal is passed to a PID torque regulator 137
which generates a requested torque signal and passes it to a summer
138.
A saturation calculator 139 estimates a rotor flux time constant
.tau.r=Lr/Rr, i.e., rotor inductance over rotor resistance and
magnetizing inductance Lm based on the equation:
where a1 and a2 are constants and R2 is temperature dependent
resistance of the rotor. For a 500 watt induction motor used in a
working embodiment of the present invention, a1=0.05, a2=0.035 and
R2=0.018@25.degree. C. It is noted that the estimates generated by
the saturation calculator 139 are used by the EM torque calculator
128.
An inertia compensator 140 receives the speed .omega.r of the
output shaft 112 of the motor 102 to generate a signal
representative of the inertia of the rotor of the motor 102 which
is coupled to the steering shaft 108 via the gear set 114. The
presence of the rotor inertia will be felt in the steering wheel
104 just as the added inertia of an air bag in the steering wheel
104. The inertia compensator 140 uses estimated rotor acceleration
which is derived by taking the derivative of the estimated rotor
speed, i.e., the speed .omega.r of the output shaft 112 of the
motor 102, and multiplies the estimated rotor acceleration by motor
inertia with the result being added to the requested torque signal
by the summer 138 to generate the torque command signal
I.sub.q.sup.e* which is passed to the power inverter and induction
motor controller 126.
Basic operating control of the motor 102, whether an induction
motor as illustrated or a synchronous reluctance motor, is in
accordance with well known operating techniques such as field
orientation control and various scalar control methods so that only
the power inverter and induction motor controller 126 of FIG. 1
will be further described herein for clarification of the invention
of the present application. In accordance with the present
invention, the induced field of the armature or rotor of the motor
102 is controlled in accordance with a speed dependent flux program
or map contained within the flux map table 122.
By using an induction motor or synchronous reluctance motor in the
EPAS system, constant excitation of the motor armature or rotor is
required via a power inverter in order to maintain the rotor flux
level active and ready for instant response. It is important to
maintain a high flux level at low speed for example to assist in
parking and other low speed maneuvers. At high speeds, a high flux
level is not required. Since a small motor of 200 to 500 watts at
the shaft can be used in the EPAS system 100 and such motors have
small rotor flux time constants on the order of 30 to 90
milliseconds, in accordance with the invention of the present
application, rotor flux is tailored in accordance with the speed of
the vehicle including the system 100.
The flux map table 122 thus associates a vehicle speed V with an
appropriate operating rotor flux level .lambda.dr for the motor 102
of the EPAS system 100 and includes at least one flux program for
that purpose, i.e., operation of the motor 102. Preferably, the
flux map table 122 is programmed to contain a number of flux
programs which depend on driver demographics and/or usage. The flux
programs can then be selected based on a given driver and can be
changed if a vehicle including the EPAS system 100 is driven by a
number of different people or for a number of different purposes or
is sold to a new owner.
When field oriented controllers or some scalar controllers are used
in the power inverter and induction motor controller 126, detuning
effects are minimized by inclusion of the saturation calculator
described above. With motor saturation accounted for, the nonlinear
flux characteristic .lambda.dr(Id.sup.e) and its impact on
magnetizing inductance Lm, on stator and rotor inductance Ls and
Lr, and on calculations based on these parameters, such as .tau.r
and Tem, are then representative of actual motor behavior.
Two exemplary flux programs are shown in FIG. 2. A vehicle which
spends most of its time in commuter service would benefit from a
more rapid transition to different flux levels as represented by
flux program B. Alternately, a vehicle used in business such as
postal delivery, police patrol, municipal utilities and the like
which do not spend any protracted time at any given speed would
benefit from higher average flux levels in the motor as represented
by flux program A.
During low speed operation, for example to perform parking
maneuvers where speeds are near zero and steering rack forces are
near or at maximum, the rotor flux is programmed to maximum. This
results in the motor 102 providing torque at a high torque/amp
value. At intermediate speeds, for example 15 miles per hour (MPH)
to 45 MPH corresponding to urban driving, the flux can be reduced
to some intermediate value as in flux program B or progressively
reduced to a minimum value as shown in flux program A. At speeds
above 45 MPH which roughly corresponds to highway driving and rural
operation, the EPAS duty cycle is low, especially for cross country
cruise and mostly straight ahead driving, the goal of the EPAS
system 100 is to reduce energy expended. For the EPAS system 100,
this translates into holding the magnetizing current to a low value
so that internal loss mechanisms in the motor 102 and power
transistors of the power inverter and induction motor controller
126 are minimized yet provide sufficient rotor flux to meet
steering needs such as lane changes, obstacle avoidance and the
like.
It should be apparent that with rotor flux reduced to approximately
25% of maximum that if high motor torque is commanded, within a
boost gain specification curve 142 of steering, that commanding
motor torque current to 100% can be handled by the inverter and the
motor. During such a transient control situation, the motor 102
responds quickly. If the transient need continues, the flux map
table 122 can be supplemented to increase commanded rotor flux by
means of a steering torque feedforward command. For example, the
flux map table 122 could include an integrator for processing the
steering torque Tstr and adding it to the value read from the flux
program currently being used.
To clarify operation of the invention of the present application, a
description will now be made of how the flux programs of the flux
map table 122 tailor steering assist torque according to speed
and/or driver preference when an induction motor is used as the
motor 102. Reference is made to FIG. 3 which illustrates in
schematic block diagram form a portion of the power inverter and
induction motor controller 126 which includes a slip calculator
144. The slip calculator 144 receives both the torque command
signal I.sub.q.sup.e* and the magnetizing or flux command signal
I.sub.d.sup.e* with a resulting slip gain being a function of these
command signals and the magnetizing inductance Lm and rotor flux
time constant .tau.r=Lr/Rr, i.e., rotor inductance over rotor
resistance, both understood to be subject to saturation effects
which can be compensated to some extent by the saturation
calculator 139 as illustrated.
For example, in a working embodiment of the invention with a 500
watt induction motor, if the induction motor parameters are
constant, with a magnetizing current Ids of approximately 40 amps
the slip gain ks=0.37 rad/s/amp and a resulting slip S.omega.e=ks *
Iq is summed with the actual rotor speed .omega.r by a summer 146
to generate the inverter electrical frequency we which is passed to
a rotator 148 which converts synchronous frame signals to
stationary frame signals. The output of the rotator 148 is passed
to a 2-phase to 3-phase transform 150. The actual slip is then the
slip that maintains field orientation for the given speed-torque
operating point of the motor 102.
With reference to FIGS. 4 and 5, the induction motor responds to
both the inverter current vector Is and the slip S.omega.e to
produce torque Tm=kt * Iq on the output shaft 112. For a working
embodiment with a 500 watt induction motor, the actual torque
constant kt was kt=0.07 Nm/Amp so that the shaft torque Tm=0.07 *
Iq Nm. If the slip angle is changed, for example by limiting rotor
flux by means of a flux program in the flux map table 122 according
to vehicle speed, then the rotor flux is constrained to be less
than the maximum air gap flux (peak flux/amp operation) and the
magnetizing component of motor current is limited resulting in
reduced overall losses and more efficient operation during highway
driving and rural operation.
The slip calculator determines slip S.omega.e using the
equation
When Ids*=40 amps for the noted induction motor, then
ks.congruent.0.37 rad/s/amp. If the motor is operating at torque T1
on a slip curve 152 at a certain motor speed .omega.r0 and more
motor torque T2 is needed, then the inverter frequency S.omega.e
jumps to a higher value S.omega.e1 such that the induction motor is
operating higher up on a slip curve 154, see FIG. 6. Similar
operation occurs for any rotor speed .omega.r and commanded torque
Tm*. Motor slip and developed torque are linked through the slip
calculator instantaneously.
By flux programming according to vehicle speed, the induction motor
slip gain characteristic is effectively constrained by the flux
programs in the flux map table 122. For example, at low speeds
encountered for example during parking, the rotor flux is set to
the maximum air gap flux possible for maximum torque per amp. In
this case, the magnetizing or flux current is not limited until
some high value, e.g., 80 amps, is reached. If maximum air gap flux
is maintained during low torque demands, e.g., during highway
driving and rural operation, then motor copper losses would be
excessive. Accordingly, the flux level is controlled according to a
flux program in the flux map table to provide sufficient motor
torque capability when needed but to hold down motor losses.
It is also possible to make the flux programs adaptive to further
modify the flux command so as to counteract temperature effects
resulting in changes in the rotor time constant and hence slip
gain. As an example, see the adaptive flux program A' in FIG.
2.
Having thus described the invention of the present application in
detail and by reference to preferred embodiments thereof, it will
be apparent that modifications and variations are possible without
departing from the scope of the invention defined in the appended
claims.
* * * * *